Use os.exec* for pipx run app execution under POSIX.

[itsayellow]

• I have added an entry to docs/changelog.md

Summary of changes

This changes how a pipx run app is executed.

For POSIX systems (AKA not Windows) it uses os.execvpe() to replace the current pipx process with the app in question. From that point on pipx is not acting as a parent process and is out of the picture.

For Windows, this wasn't a reliable method, so for Windows subprocess.run() and sys.exit() are used to approximate os.exec*.

Because now running an app never returns control to pipx, modifications to deleting a .cache venv when under --no-cache operation had to be made. Previously this was done after running the app. Now, before running the app, a file named pipx_expired_venv is placed inside the venv as a signal to future pipx run invocations to delete this venv during _remove_all_expired_venvs().

Testing was changed to accommodate the new behavior of exiting with an exit code instead of returning via return. The exit code is verified to be correct as needed.

Closes #506

Incidental changes

I renamed util.run() to util.run_verify() to better describe what it is and why it is different from run_subprocess().

Test plan

This should be an internal change to pipx run, and should only be cleaner with respect to certain cases, such as how signals are processed as described in #506 for example. Otherwise pipx run should execute commands with no obvious differences than before.

pipx run pycowsay Mooooooo!

[gaborbernat]